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Material-Specific Guide

MEMS & Piezoelectric Device Preparation

Metallographic preparation for microelectromechanical (MEMS) sensors and piezoelectric (PZT) devices. Both are dual-material systems combining hard ceramic or silicon with soft metallized layers, solders, and packaging. Procedures run slower than typical bulk-material prep to protect the metallized features and the ceramic-metal interfaces.

Table of Contents

Introduction

MEMS devices and piezoelectric (PZT) elements are the standard examples of dual-material metallographic prep. Both combine a hard ceramic or silicon body with soft attached features: metallized contacts, wire bonds, solder joints, sputter coatings, polymer packaging. The hardness differential between substrate and feature can span 50× or more, which makes them harder to prep than either material on its own.

This guide is the reference for both device families. For printed circuit boards and packaged silicon chips at the assembled-board level, see the PCB & Chip Preparation guide. For semiconductor wafer prep (silicon, GaAs, AlN substrates as raw substrates), see the Semiconductor Substrate guide.

How this guide is organized

MEMS and PZT have similar prep philosophies but different procedures. Both use a slow 100/100 RPM head/base speed (vs. the typical 200/200 for bulk samples) to avoid damaging metallized features. Both use castable cold mounting. The differences are in initial grit, polish abrasive selection, and number of steps.

MEMS uses a 600 grit (P1200) SiC paper initial plane and a 2-step polish (1 µm DIAMAT with SIAMAT, then SIAMAT CMP). PZT uses a finer 1200-grit initial plane and adds a NANO-W nanometer-alumina step between the 1 µm diamond and final colloidal silica. The extra step on PZT compensates for the much smaller feature sizes on modern piezoelectric devices.

The 100/100 RPM rule: Both MEMS and PZT run at 100 RPM head and 100 RPM base, not the standard 200/200. The lower speed reduces dynamic load on metallized layers, solder joints, and wire bonds, which can fracture or smear at higher RPM. Don't substitute faster settings; the prep tables below are calibrated to this speed.

MEMS Devices

Microelectromechanical systems combine mechanical structures (beams, membranes, gears, cantilever resonators) with electronic interconnects on a single chip. Components range from 1 µm to 0.1 mm. Found in automotive accelerometers and gyroscopes, medical implants (cochlear implants, pacemaker sensors, infusion-pump valves), inkjet print heads, DLP display chips, and inertial measurement units. Cross-section metallography is the standard tool for failure analysis and design verification.

Prep risks: The dual-material nature is the dominant challenge. A MEMS cross-section may contain silicon (~1000 HV), aluminum (~30 HV), gold bond (~25 HV), nickel intermediate layer (~250 HV), silicon nitride passivation (~1500 HV), and a soft polymer encapsulant within the same view. Any one of these will polish differently than the others; the goal is to remove material at equal rate while preserving planarity. The second risk is gold-wire-bond pullout: gold wires bonded to aluminum pads are weakly anchored and can lift during grinding if force is too high.

Sectioning: MEMS

  • Blade: Diamond wafering blade, medium grit, low concentration (use only if the device is large enough to require sectioning; many MEMS samples are submitted intact for direct mounting).
  • Wheel speed: 200-300 RPM precision wafering saw.
  • Feed rate: 2-5 mm/min. The fragile metallized features and wire bonds dictate slow controlled feed.
  • Cooling: Continuous water-based cutting fluid.
  • Orientation: Document the cross-section plane relative to the active device area. Many MEMS analyses target specific features (a particular cantilever, a single transducer cell), and orientation is critical.

Mounting: MEMS

  • Castable epoxy or acrylic. Compression mounting heat will damage thermal-sensitive features and stress wire bonds.
  • Vacuum impregnation if open MEMS cavities (resonator gaps, fluid channels) are present.
  • For conductive-resin SEM analysis, use carbon- or copper-filled compression resin only on robust MEMS samples. Most MEMS analyses use a conductive paint bridge after castable mounting instead.

Grinding: MEMS

  1. 600 grit (P1200) SiC paper, water, 5-10 lbs, 100/100 RPM, until plane.

Single grinding step. The 100/100 RPM and 600-grit combination removes the sectioning damage without dislodging metallized features. Don't skip ahead to polishing without confirming the surface is plane and that no chips are visible at 100×.

Polishing: MEMS (2-step)

  1. 1 µm DIAMAT diamond on TEXPAN polishing pad with SIAMAT colloidal silica, 5-10 lbs, 100/100 RPM, 3-5 min.
  2. SIAMAT colloidal silica on BLACKCHEM 2 polishing pad, 5-10 lbs, 100/100 RPM, 3-5 min.

The combined DIAMAT plus colloidal silica step is unusual and intentional. The diamond does the mechanical cutting while the colloidal silica simultaneously removes diamond damage via chemical-mechanical action, eliminating the need for a separate intermediate step. The final SIAMAT-only step on BLACKCHEM 2 clears any remaining damage on the silicon and silicon nitride phases.

Piezoelectric (PZT) Devices

Piezoelectric devices generate voltage when mechanically deformed and vice versa. Standard composition is lead zirconate titanate (PZT), processed at high sintering temperatures. Found in medical ultrasound transducer arrays, sonar elements, industrial ultrasonic cleaners, automotive knock sensors, inkjet print heads (overlap with MEMS), and fuel-injection piezo actuators. Modern piezoelectric arrays often have feature sizes below 10 µm, requiring much finer prep than MEMS bodies.

Prep risks: Like MEMS, the dominant challenge is dual-material handling. A PZT cross-section may contain the lead-zirconate-titanate ceramic body (~600-800 HV), silver-paste electrodes, soft solder joints (Sn-Pb or Pb-free, ~10-15 HV), polymer packaging compound, and copper or brass packaging structures. The ceramic body fractures conchoidally if struck, and the small feature sizes on modern arrays mean that any preparation artifact at the µm scale is large relative to features. The principal grinding hazard is lead from the PZT itself: lead dust and lead-bearing slurry must be captured under wet conditions, with local exhaust ventilation and appropriate PPE per the material safety data sheet.

Sectioning: PZT

  • Blade: Diamond wafering blade, medium grit, low concentration.
  • Wheel speed: 200-300 RPM precision wafering saw.
  • Feed rate: 2-5 mm/min.
  • Cooling: Continuous water-based cutting fluid.

Mounting: PZT

  • Castable epoxy or acrylic resin. Compression mounting can crack the ceramic body and stress solder joints.
  • Vacuum impregnation recommended for PZT arrays with bond-pad voids or trapped flux.

Grinding: PZT

  1. 1200 grit (P4000) SiC paper, water, 5-10 lbs, 100/100 RPM, until plane.

Finer initial grit than MEMS (1200 vs. 600). Modern PZT array features can be small enough that even 600-grit SiC damage extends into active device regions.

Polishing: PZT (3-step)

  1. 1 µm DIAMAT diamond on GOLDPAD polishing pad, 5-10 lbs, 100/100 RPM, 3-5 min.
  2. 0.05 µm NANO-W nanometer alumina on BLACKCHEM 2 polishing pad, 5-10 lbs, 100/100 RPM, 3-5 min.
  3. SIAMAT colloidal silica on BLACKCHEM 2 polishing pad, 5-10 lbs, 100/100 RPM, 1 min.

The 3-step polish ending in NANO-W alumina and then a brief SIAMAT CMP gives the cleanest surface for high-magnification SEM imaging of modern PZT arrays. The NANO-W step suppresses relief between the PZT ceramic and the soft solder/silver-paste features; the brief SIAMAT CMP removes the last sub-surface damage on the ceramic phase.

Sputter coating for high-magnification SEM: PZT cross-sections are typically imaged at 30,000-200,000× for failure analysis. At those magnifications, a 5-10 nm Au or Pt sputter coat is mandatory to give consistent SEM contrast across the dielectric ceramic, conductive electrodes, and polymer packaging. The first PACE reference image at this stage is typically the sputter-coated cross-section under SEM.

Other Electronic Ceramics & Solders

Beyond MEMS and PZT, several other electronic ceramic systems and the solder joints used to assemble them require metallographic preparation in failure analysis and quality control work. Three are documented in PACE's source procedures: nickel-zinc ferrite (magnetic ceramic), multilayer ceramic capacitors (MLCs), and solder joints on both ceramic and non-ceramic substrates.

Nickel-Zinc Ferrite (Ni-Zn Ferrite)

Magnetic ceramic of the spinel ferrite family, composition NiZnFe2O4. Used in RF/microwave inductors, transformer cores, and antenna components where ferrimagnetic properties and controlled permittivity matter. Friable and brittle; edge chipping during sectioning is the dominant prep risk.

Prep procedure: Diamond wafering blade, medium grit, low concentration. Castable epoxy or acrylic mounting; vacuum-assisted impregnation is recommended for the porosity inherent to sintered ferrites. SiC paper progression 240 grit (P280) through 1200 grit (P4000) at 200/200 RPM, then 1 µm DIAMAT diamond on ATLANTIS pad with SIAMAT colloidal silica for 5 min at 200/200 RPM, then 0.05 µm nanometer alumina on NAPPAD as the final step. The extended 5 min DIAMAT/SIAMAT step is the key procedural difference; ferrite grain pullout is suppressed by the chemical-mechanical action of colloidal silica.

Multilayer Ceramic Capacitors (MLCs)

Stacked-layer ceramic capacitors using BaTiO3 dielectric layers alternating with thin metallic electrodes (Ag-Pd or Ni). Used everywhere in modern electronics for high-volumetric-capacitance bypass and filtering. The defining metallographic challenge is delamination at the ceramic-metal interfaces during preparation, and void preservation in the dielectric layers.

Prep procedure (lapping film progression): Vacuum-assisted castable epoxy or acrylic mounting to fill any void content and prevent delamination during grinding. The MLC procedure skips traditional SiC paper grinding in favor of alumina lapping films:

  1. 45 µm alumina lapping film, 100/100 RPM, planar to flat surface.
  2. 30 µm alumina lapping film, 100/100 RPM, planar.
  3. 15 µm DIAMAT diamond on POLYPAD with DIALUBE Purple extender, 100/100 RPM, 3 min.
  4. 6 µm DIAMAT diamond on GOLDPAD, 100/100 RPM, 3 min.
  5. 1 µm DIAMAT diamond on ATLANTIS pad with SIAMAT colloidal silica, 100/100 RPM, 2 min.

The lapping film progression (45 / 30 µm alumina) is gentler than equivalent SiC paper and preserves the thin electrode layers. Skipping a step in this progression causes interface relief or delamination that no subsequent polish can recover.

Solder Joints (on Ceramic Substrates)

Solder joints on ceramic substrates (alumina or aluminum nitride DBC boards, MLC packages, hybrid microelectronic packages). The hardness mismatch between soft solder and hard ceramic produces edge rounding at the interface; the polish technique focuses on preserving the joint geometry.

Prep procedure: Diamond wafering blade, medium grit, low concentration. Castable epoxy or acrylic mounting with vacuum-assisted impregnation. Polish through diamond lapping films:

  1. 30 µm diamond lapping film with POLYLUBE extender, 200/200 RPM, 2-3 min.
  2. 15 µm diamond lapping film with POLYLUBE, 200/200 RPM, 2-3 min.
  3. 3 µm diamond lapping film with POLYLUBE, 200/200 RPM, 2-3 min.
  4. 1 µm DIAMAT diamond on ATLANTIS pad with DIALUBE Purple extender, 200/200 RPM, 1 min.
  5. Final colloidal silica on MICROPAD, 100/100 RPM, 1 min.

Solder Joints (on Non-Ceramic Substrates)

Solder joints on copper, nickel-plated, or other metal substrates. Standard SMT (surface-mount) and through-hole assemblies. The challenge here is abrasive embedding in the soft solder, which produces scratches that look like fatigue cracks.

Prep procedure (gentler than ceramic-substrate version): Castable epoxy or acrylic mounting. Polish through finer diamond lapping films at slower RPM:

  1. 15 µm diamond lapping film with POLYLUBE extender, 100/100 RPM, planar.
  2. 3 µm diamond lapping film with POLYLUBE, 100/100 RPM, 2 min.
  3. 1 µm DIAMAT diamond on ATLANTIS pad with DIALUBE Purple extender, 100/100 RPM, 1 min.
  4. SIAMAT colloidal silica on MICROPAD 2 pad, 100/100 RPM, 1 min.

Skip the 30 µm lapping film step that ceramic-substrate solder joints use; the softer metal substrate doesn't need that aggressive a starting point and the coarse film embeds in the solder rather than removing it.

Lapping film vs cloth-mounted abrasive: Electronic ceramic preparation leans heavily on diamond lapping films instead of cloth-mounted abrasives. Lapping films are rigid and conformable to flat samples but unforgiving of high spots, which means they preserve the planarity that thin-film electronic features require. Cloth-mounted abrasives roll over thin features and round their edges. For the MLC and solder procedures above, do not substitute cloth-mounted abrasives even if comparable grit sizes are available; the rigidity of the film is the point.

Imaging & Contrast

Both MEMS and PZT are imaged as-polished. Chemical etching is rare and risks attacking metallized layers or solder joints preferentially.

  • Brightfield optical: Default for initial inspection. Reveals metallized features and ceramic boundaries clearly on properly polished samples.
  • DIC: Useful for finding sub-surface damage in the ceramic phase and slight relief between phases.
  • Dark-field: Best for finding cracks in the ceramic body and delamination at metallization interfaces.
  • SEM with sputter coating: Standard for modern piezoelectric arrays with sub-10 µm features. 5-10 nm Au or Pt sputter coat lifts contrast at high magnification.
  • EDS: Useful for identifying composition of solder joints, intermetallic layers between bond pads and wire bonds, and contamination signatures in failure analysis.

Troubleshooting

Gold wire bonds pulled out during MEMS prep

Cause: Grinding force too high or wheel speed above 100 RPM; soft Au-to-Al bond fractures under dynamic load.

Fix: Confirm 100/100 RPM (not 200/200). Reduce force to the lower end of the 5-10 lb range. Use vacuum impregnation before grinding if the bond is critical to the analysis.

PZT ceramic cracking during mounting

Cause: Compression mounting was used instead of castable, or the castable epoxy was not given enough cure time before initial grinding.

Fix: Use castable cold mount only. Cure for the manufacturer-specified time at room temperature; don't accelerate by heating. PZT is also vulnerable to thermal shock during cure of fast-cure resins.

Solder joint smearing across PZT array features

Cause: Soft solder flows under polishing pressure, smearing into the spaces between PZT array cells.

Fix: Reduce polish force at the 1 µm DIAMAT step. Switch to GOLDPAD if currently using a softer pad; the firmer GOLDPAD limits solder flow. Confirm 100 RPM, not 200.

Silicon nitride passivation layer chipping on MEMS edges

Cause: SiC paper too coarse (using 240 or 320 grit instead of 600), or sectioning damage from a coarse blade propagated into grinding.

Fix: Confirm 600 grit (not coarser) for MEMS plane grind. Re-section with a medium-grit blade if sectioning damage is visible.

Featureless brightfield image on PZT at high magnification

Cause: Optical contrast insufficient for sub-10 µm feature sizes; the dielectric ceramic and polymer packaging have similar reflectivity in brightfield.

Fix: Sputter coat with 5-10 nm Au or Pt and switch to SEM imaging. Brightfield optical is for survey work; SEM is standard for PZT array analysis above 5000×.

Relief between ceramic and metallized features after polish

Cause: Hardness mismatch between hard ceramic body and soft metallized features. Polish removed soft phase faster.

Fix: For PZT, confirm the NANO-W step ran for the full 3-5 min; the alumina abrasive flattens this. For MEMS, extend the SIAMAT colloidal silica step. If relief persists, reduce 1 µm DIAMAT step force.

Additional Reading

  • Zipperian, D.C. Metallographic Handbook. PACE Technologies, Tucson, AZ. House reference for MEMS and PZT prep procedures.
  • Madou, M.J. Fundamentals of Microfabrication and Nanotechnology, 3rd ed. CRC Press. Comprehensive reference on MEMS device architecture relevant to interpreting cross-sections.
  • Bhushan, B., editor. Springer Handbook of Nanotechnology. Springer. MEMS and piezoelectric sections cover materials and processing.
  • Jaffe, B., Cook, W.R., Jaffe, H. Piezoelectric Ceramics. Academic Press. Foundational reference on PZT processing, composition, and properties.
  • Kasap, S. and Capper, P., editors. Springer Handbook of Electronic and Photonic Materials. PZT and ferroelectric sections relevant to packaging analysis.
  • ASTM F1378. Standard Practice for Visual Inspection of Microelectronic Wafers and Devices.
  • ASTM E1245. Automatic Image Analysis (porosity and feature-size measurement).
  • ASM Handbook, Vol. 11: Failure Analysis and Prevention. ASM International. Electronics failure-analysis chapter relevant to both MEMS and PZT.

Explore More Procedures

For assembled PCBs and packaged silicon chips, see the PCB & Chip guide. For raw wafer-level substrate preparation (silicon, GaAs, AlN), see the Semiconductor Substrate guide.

PCB & Chip Preparation Semiconductor Substrate Medical Device Applications

Related Guides

→ PCB & Chip Preparation → Semiconductor Substrate → Medical Device Applications → Failure Analysis → Microstructural Analysis